Information on size and velocity of spherical objects including particles, droplets, bubbles, etc., is important for numerous applications in various industries. These applications include, for example, fuel spray combustion analysis and control for the automotive industry, inhaler manufacturing for the pharmaceutical industry, household spray manufacturing, agricultural pesticide irrigation, aircraft icing analysis and control, spray nozzle manufacturing, atmospheric aerosol analysis, and various combustion related applications.
Normally, a laser light scattering interferometry technique is used to determine the size and velocity of spherical objects, such as particles, drops, bubbles, etc. According to this technique, spherical objects under the test pass the intersection point of two crossed laser beams generated from the same laser. The light scattered from the spherical object, as it passes the intersection point of two laser beams, forms a fringe pattern on the detector. The spatial period of the interference fringe pattern generated by the spherical object, as it passes through the intersection point of two crossed laser beams, is used to determine the particle size and one velocity component in the plane of two crossing laser beams. In order to obtain a reliable and accurate measurement, it is important to provide a stable interference pattern on the detector. The shape and spacing of the fringes formed by the scattered light depend on the angle between the incident laser beams, their wavelength, the direction of the light collection, and the particle diameter. In particular, the quality of the interference pattern on the detector depends on the pointing stability of two crossing beams and on the coherence length of the laser. For example, a longer coherence length of a laser provides a better quality interference pattern. Further, to obtain a substantially high signal-to-noise ratio of the signal generated from the scattered light, the probing laser needs to output enough optical power to compensate for losses in the optical system.
Certain prior art light scattering interferometry optical systems use a gas laser. A gas laser, however, typically is bulky, consumes a lot of electrical power, and requires a cooling system involving water or forced air. Accordingly, optical systems that use such lasers are typically bulky and sophisticated. Such systems usually consume a lot of electrical power and involve frequent realignment to ensure measurement reliability and accuracy. Highly skilled professionals are usually required to operate such systems. Such prior art systems typically do not reside in a single compact, portable package. Such prior art systems are typically not used for daily testing and quality control in highly-paced manufacturing and field environments.
Other light scattering interferometry prior art optical systems used to measure particle size and a velocity component employ a solid-state laser. The output of the solid-state laser is coupled to a polarization-preserving single-mode optical fiber to direct two crossing laser beams onto the spherical particle under the test. In such prior art systems, the light scattered from the particle is collected into the multi-mode optical fiber that brings the scattered light to photodetectors to measure the scattered light signal.
FIG. 1 is an illustration of a prior art optical system using a gas (He—Ne or Argon) or solid-state laser. The laser has the output light coupled into single-mode polarization preserving fibers to direct two crossing beams onto the particle. Multimode fiber optics receive the scattered light from the particle. As shown in FIG. 1, the output of the laser 110 is connected through the coupler component 120 to a single mode polarization preserving fiber 130. The optical fiber output head 140 is positioned on the rail to direct crossing beams onto the particle 190. The scattered light from the particle is collected by the input head 150 of a multimode fiber, which is positioned on the rail at a predetermined angle relative to the incident light, and directed by the multimode fiber 160 to the receiver module 170. The signal from the receiver module 170 may further be processed by a signal processor to obtain the information on the size and velocity component of the particle.
The use of movable, fragile optical fibers can, however, put the stability of the optical alignment of the system at risk. In particular, the alignment of the laser beam to a single mode polarization preserving fiber having a small fiber diameter from 2 to 5 micrometers (“um”) is crucial to the stability of the intersection point of incident laser beams to provide accurate and reliable measurements. In addition, losses of the laser light in the coupling at the output of the laser typically substantially reduce the overall laser efficiency, leading to higher electrical power consumption by the system. Further, losses of the scattered light from the particle in the coupling into the multimode fiber typically significantly reduce the overall sensitivity of the receiver, leading to the system's substantially reduced operating range. Furthermore, typically time-consuming high-precision assembly of such optical systems is required to couple the light into the optical fiber.